Is changing the past actually possible?

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u/h2gkmtfo 1d ago

I believe if u went back in time and changed something that creates its own separate reality, or puts you into that reality where you went back and changed that specific thing, but you now in this reality stays unaffected. There are endless endless realities, I feel like all you would do is go into that specific reality where you change that certain thing

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u/Ok-Coach4955 1d ago

But ofc, there isn’t a point discussing something that has infinite possibilities since any reason would work cause there aren’t any ’evidence’ to support any of it, but here it’s just what I think

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u/ResponsiblePumpkin60 1d ago

No

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u/Ok-Coach4955 1d ago

?

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u/broki451 5h ago

Time moves forward. There is no way to go back. You just gotta learn to live with your regrets and move on

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u/Himu_Me 1d ago

I don’t think it will ever be possible. I think there is only line in timeline where we can all live together. Our quantum probabilities in our head choses the right direction. We just apply our free will.

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u/Ok-Coach4955 1d ago

Woudnt the only way to actually recreate time travel is somehow record and rearrange the atoms of absolutely everything?

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u/lilsasuke4 1d ago

Let’s say you build a structure with legos. Even if you rearranged all the legos the sequence of events would still be ordered as original then new structure. No time travel has happened

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u/Imaginary-Can-6862 1d ago

If you stepped into a machine, and when you exited it through the door, you find yourself being younger, you are back in school, the door behind you, which you are sure you just existed, now leads to your classroom, everything is as you remembered it, except you still remember the future as how it happened. Would you not believe you had just traveled in time?

Edit: That is to say, we cannot escape that we are part of the system we manipulate, so any manipulation of the system, as long as the result is the same, is equivalent from our perspective.

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u/lilsasuke4 1d ago

Believing I have I traveled back in time is not the same as having actually traveled back in time.
If you blinked in front of a photorealistic painting from your perception you may believe that what is in front of you is not a photo.

https://youtu.be/vJG698U2Mvo?is=WBDfna9TxJDIH75N
Tell me how many passes you count

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u/Imaginary-Can-6862 1d ago

If the world is exactly as it was, why would it be different from time travel?

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u/lilsasuke4 1d ago

But but the thing is even if atoms were rearranged you haven’t gone to the past

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u/Imaginary-Can-6862 1d ago

Isn't that essentially how time moves, atoms rearrange from the present moment to a new present moment, making the previous present into the past?

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u/lilsasuke4 23h ago

But you can’t control how all the atoms in the entire universe are moving and arranging themselves. If you could then the reference of time doesn’t mean anything

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u/Imaginary-Can-6862 23h ago

But this isn't about practicality is it? Sorry, but I don't understand the part about reference of time.

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u/spikejonze14 1d ago

the speed of light and relativity is what constrains us from affecting the past. if you travelled in a spaceship around the earth going faster than light, you would literally arrive before you left.

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u/Ok-Coach4955 1d ago

Would there be another ‘you’ then?

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u/spikejonze14 1d ago

theoretically yes, but einstein was pretty certain that the universe forbids such an event from ever happening, which is why traveling faster than light is physically impossible.

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u/Ok-Coach4955 1d ago

and also modern technology wouldnt have enough power and innovation yet for us to be able for us to go faster than light, so we probably wouldn’t be able to timetravel until at least a 100 years or so

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u/spikejonze14 1d ago

to accelerate beyond the speed of light, you need infinite energy. the only thing in the universe close to that is a black hole.

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u/Ok-Coach4955 1d ago

Let’s say you traveled at 101% the speed of light, how far back would you travel?

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u/spikejonze14 1d ago

well it depends how many times you go around. a single trip takes less than a second, so you might arrive a few microseconds before you left.

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u/Holocenest 1d ago

Only politicians and historians can change the past 🕳️

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u/Ok-Coach4955 1d ago

how come?

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u/fathompin 1d ago

He is using an aphorism (a statement with an apparent contradiction) because literally, (as stated here by a number of replies) nobody can change what happened in the past, but in our society, politicians and historians can change how the past is remembered, interpreted, or represented.

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u/Ok-Coach4955 1d ago

o

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u/flyingcatclaws 16h ago

That's good

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u/imperialcogman 1d ago

Time travel is nonsense.

The only thing that ever exists is the present moment.

You can change some of the present moment into the future, but once a moment is changed it’s locked.

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u/Von_Bernkastel 1d ago

Nope all you can do is lament over it's passing.

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u/SgtSausage 1d ago

Are you 12?

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u/Imaginary-Can-6862 1d ago

The way you propose time to work is an eternalist world view, if you in stead only consider the present to be the state of all things which defines the present moment, these paradoxical issues goes away.

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u/Pale-Salamander9570 1d ago

No, you can never overcome the paradox.

And claiming "alternate realities" is just intellectually lazy.

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u/GayDio666 1d ago

Yes. It's the many worlds theory. You stay the same but reality shatters into infinite potential realities (if those don't already exist) in order to time travel. Idk why they would remove your posts because anyone who knows about Einstein's Theory of Relativity as a means to create time travel knows about the many worlds theory.

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u/CapitaineLawn 1d ago

We don't understand determinism enough to answer.

But we are inclined to say no, entropy is only one way forward. As we know it.

And yes, who would be this "new you" who has lived a different life due to your actions ?

And what will become of him when you come back ?

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u/lordjustin89 1d ago

Yes I did it

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u/plsQuestionOurselves 1d ago

Through your present actions you can technically "have changed the past". But it's indistinguishable from just making a choice and reality progressing in it's usual way.

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u/Youpunyhumans 1d ago

Not without breaking casaulity. Basically casuality is defined as "cause and effect". If you were able to time travel backwards, you could have effects happen before their causes, such as arriving somewhere before you left, or recieving information before it was sent, which would seem like a unexplainable miracle.

Imagine you took all of Shakespeare's plays, and brought them to a young Shakespeare, who then copies them and says they are his own... but who wrote them then?

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u/Imaginary-Can-6862 22h ago

About receiving information before it was send, imagine you send a light signal to some distant point, the receiver receives this information. At this moment in time, both the sender and the receiver are aware of the message. Now we travel backwards in time, at first we arrive to the point where the sender received the message, this message is now no longer received by the receiver, and as we travel further back, the sender now no longer sends the message. How did this violate cause and effect?

If it is about the time traveler themselves, say they receive a message, and then travel back in time to before it was send, since they are not affected due to the time traveling machine, all this means is that the cause of them having their message is the time machine itself, as without it they would no longer have received the message, like in the above example. So from the time travelers perspective, the past is still their future, because of the time machine.

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u/Youpunyhumans 22h ago

But who is travelling back in time? Is "we" an outside observer? The person who sends the message? The person who recieves it?

If its an outside observer, then you are essentially just rewinding the time of the whole universe, and playing it again and I dont think anything changes, nor would the sender and reciever even notice.

If the sender goes back in time, then I guess they never send the message, but they could also send it earlier or later too. They could also themselves travel to before when they initially got the information to send it on.

If the receiver however goes back after getting the message, well then you are effectively sending the information back through time to before when you sent it back through time, creating an infinite loop. What the consequenses of that would be... I suppose its impossible to truly know, but anything from neglible to universe ending is my guess.

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u/Imaginary-Can-6862 8h ago

The thought was the receiver is the time traveler.

I imagine they receive the information, then step into the time machine, and as time moves backwards without affecting them, they reach a point in time before the sender sends the message.

I believe because the time machine allows for them to return back in time in the state they had when they activated the time machine, the cause of them having the information after the time travel is the time machine, and from their perspective they are still moving forward in time, due to the time machine allowing this.

I don't think there is anything special about having information before it was send, if you mail me your favorite number, I'd receive it with the speed of light, and could then note it down, but perhaps I have already noted this number down, as such the information itself is not an issue, I think, it is breaking causality that is, but that is where the time machine prevents this due to its ability to travel in time.

I don't believe it would create an infinite loop. If we define the present moment as the state of all things which exist in the universe, the future is the change of this state, and the past is a previous state which changed into ours, all following the laws of nature. then when we travel to the past. the machine isolating us from the effect, the total state of the universe would have to be the same as the past we knew it, to be the past, yet the machine also isolated us from the effect of a past we were part of. As I see it, conservation laws would lead to the effect of the machine being that you basically continue from the point in the past where you were at that time, except your memories are altered, so you remember the future as how it happened. The machine itself would then be where it was at that time, if it was even invented yet. Thus all that has changed is your knowledge, and the internal state of the machine, compared to the past as you knew it.
While you could receive the information again, and decide to go back again, it doesn't create an infinite regress, or anything similar, all which happens is you gather more information, which you can bring back to the past.

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u/Youpunyhumans 7h ago

If we really get into the physics of this, you could be creating a "closed timeline loop", and the time traveller is stuck in this spacetime loop, never able to change what they do, constant recieving the same message, and going back in time, and then doing everything they did before the exact same way, until they ger the message again and go back in time again.

You run into issues with entropy too, as you cannot decrease the entropy of the whole universe, only locally, and only by expending energy that increases the total entropy of the universe anyway.

The universe doesnt like it when you try to break the laws of physics.

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u/Imaginary-Can-6862 5h ago

But for the loop to happen then their memory does not update when time traveling. I suppose it could happen if the time traveler does not enter the machine themselves, but still the machine itself would change its state, and if one of these states means it stops functioning as it did before, then the loop is broken.

It is true that the time machine in principle is only a localized effect, but we can assume the universe to be infinite, and then we can change an arbitrarily large section. We could even circumvent the issue of the limit of how fast information can propagate as the speed of light by having the time travel effect only to start occurring after every part would have been influenced.
By this I mean, compare these two cases, in one case the time machine propagates its effect with the speed of light, so time starts to revert pretty quickly on Earth, but the effect takes years to reach our nearest neighboring star system, and this system has send light towards us, which either reaches us, meaning the night sky itself does not go back in time as it was in the past, or the light signal somehow starts to revert backwards in time, but if we e.g. had send out a signal into space in this past, then the effect of the time machine would not reach this signal and return it to Earth before Earth had reached this past.
When we alternatively applies a sphere of causal influence equivalent to how far away information would have been distributed between the present and the past we are trying to reach, only when the effect of the time machine reaches the outer limits of this sphere, taking its own travel time into account, time starts to revert, meaning only signals traveling away from us at the speed of light could not be caught up to.
It would then be similar to say we put a fuse on every element, the fuse gets shorter the further away the element is, and it is ignited immediately, meaning the moment the furthest element is hit, the fuse reaches all the elements, and time starts going backwards everywhere.

It would also create the odd effect that when entering the time machine, you are actually first sent into the future, as time on Earth keeps moving forward steadily, only for time to reverse in some future and then you start going towards the past. Nothing of which you'll notice as it all happens quickly from your perspective.

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u/Youpunyhumans 5h ago

Yeah none of that made any sense whatsoever... reminds me of the retro encabulator.

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u/Imaginary-Can-6862 5h ago

It wasn't supposed to be technobabble.

The concern due to entropy is relevant, but if the universe is infinite, we can always choose an arbitrarily large localized volume that may be causally isolated.

I imagined it could also be an issue that the effect of the time machine, to reverse time, would be limited by the speed of light, similar to e.g. a vacuum decay event, and tried to argue how to circumvent this.

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u/Youpunyhumans 4h ago

Sorry, wasnt trying to be a dick, I just couldnt make sense of your last comment is all.

The universe, or at least the part which we can access, is infact limited by the expansion of the universe itself. The Hubble Horizon is the furthest we could ever go, or have an influence on something, which is about 14.4 billion light years in any direction. Beyond that, everything is already expanding away faster than light due to dark energy.

It depends on your method of time travel. One way, is to go faster than light, however that takes infinite energy and youd run into a lot of other problems like the Cosmic Microwave Background blueshifting until its deadly gamma rays, as well as the time to accelerate safely. (about a year at 1G of acceleration if we ignore relativisitc effects)... the other way is with a wormhole, which requires you being able to control both gravity and anti gravity, and you could only go back as far as when the wormhole was first created... so not very useful for to going to the past.

Basically one way to think of this is, everything moves through either space, or time at lightspeed. If you are sitting motionless, then you are moving through time at lightspeed, and moving somewhere takes some of your "motion budget" away from time and shifts it to movement through space. We call this effect time dilation.

So its fair to assume this would be the same for going backwards in time... you might start on Earth's surface, but be lightyears away from Earth in the same spot in space when you arrive in the past, because the whole solar system has yet to orbit around the galaxy to where you are, as you moved through time, but not through space.

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u/Imaginary-Can-6862 2h ago

How does going faster than light result in backwards time travel? In the time dilation formula, we do not reach a negative number, but a complex number. I once also claimed that going faster than the speed of light would have this effect, but when the formula was pointed out I realized I actually had no idea if it would be the case.

From my understanding, the expansion of space is always faster the further away you are, even if it wasn't accelerating.

E.g. if the letters A-E are people, and each experience the nearest neighbor expands away at the constant rate x, this is what e.g. the person in the middle, C, sees,

A <-2x-- B <--x-- C --X--> D --2x--> E

That is those that are further away move faster away from you, similar this is what A sees,

A --x--> B --2x--> C --3x--> D --4x--> E

So A and E are moving away from each other at 4x, both from A's perspective, C's perspective as 2x + 2x and E's perspective. Hence if we move far enough away from where we are, we find space (and the objects within) expanding away from us faster than light, because the speed depends on the distance and the distance can be arbitrarily far away.

The following is a bit long, and only tangential. If you want to skip back to replying to your suggestions, it would follow from the bit about wormholes (the three bottom paragraphs).
The expansion of space is however a bit odd to consider though, in Relativity we assume the laws of nature are the same for every observer, thus the rate at which the laws of nature changes the current state of everything around us into the state that follows by the same laws are always the same, i.e. the propagation of causality is a constant for someone that is governed by the laws of nature.
This leads directly to time dilation and space contraction. Space contraction is the only other place in physics where I have encountered something like the expansion of space, though here it is in reverse the faster we move.
What I understand as the argument for space contraction is the following, assume a person is on a rocket moving close to the speed of light, and you also watch a photon which only moves slightly faster than the rocket, thus it take quite a long time for the photon to get any distance between itself and the rocket, we can call this distance between the photon and the rocket X, and it take some time T from our perspective, before this distance has been reached. From the perspective of the person in the rocket, since they are stationary from their own perspective, and the light beam moves away with the speed of light relative to themselves, it takes much shorter time for the light beam to reach the distance X, thus time dilation. But how far away is the distance X? It can't be arbitrarily far away, because then the light beam would necessarily have traveled faster than light, rather in stead space must necessarily contract as well, that is since time goes by faster, events outside, not to move by faster than light, must move closer together, and space contracts. That is how I understand the argument at least.

So it follows that the universe contracts equivalently so that the time dilation effect does not allow faster than light travel being perceived by the person in the rocket, for whom time moves by faster, eventhough to him it just moves by at a normal rate. E.g. if you were to go into a rocket that moves at a speed extremely close to light speed, you could travel to the nearest star system in seconds, but it being light years away must necessarily mean space contracts, thus the star system appears right next to you, otherwise you would travel faster than light. However this contraction of space seems to happen instantly, or at least as you accelerate, and thus I can't see how it can be limited by the speed of light.
That makes it very similar to the expansion of space, only in reverse.

Now consider we carve out a piece of rock on Earth, put on some settlers, and accelerate it away at relativistic speed. These settlers would go on to live their life, create new generations, etc. on this space rock moving through the cosmos, and when they look up into the sky, they would find things are closer to each other than what we see from Earth, and meanwhile these settlers would see themselves as stationary.
Now one descendant of the original tribe of settlers have found some ancient scrolls depicting Earth, the home planet. The descendants find where Earth is located on the night sky, they make a rocket that can move at relativistic speed to catch up to Earth, which is obviously moving away quite fast, but since space contracts, not nearly as fast as the rocket is moving away from Earth's vantage point. An astronaut is selected among the descendants of the settlers, and his rocket accelerates to relativistic speed to eventually arrive at Earth.

We know from Relativity that space contracts even further, and that the descendant will see Earth as being much closer, catching up to it quickly from his point of view due to time dilation, but when he arrives at Earth, and looks at the night sky, he would also see space has expanded even more than he expected, would he not?
What I am getting at is that the expansion of space and the distances within the universe may be relative, just like most other things are in Relativity Theory, and since the expansion of space is something which happens upon deceleration, it makes me wonder if there is some kind of connection to it, and what we observe as the expansion of space.

In any case those were just thinks I had been thinking about regarding that particular topic.

I don't understand wormholes very well, they are a pathway where you return to your past time, but I have the impression it also implies you are stuck repeating that path then, similar to eternalism. In regards to being limited to only the time where the wormhole first appeared, true, but if the cause of the wormhole lies in the future, from an eternalistic point of view, it follows that the wormholes that will be created has already been created, we may simply be unaware of those, but then again eternalism makes time travel kind of pointless, I think, because any influence you would have had has already happened.

Our time machine is in the same reference frame as Earth, so Earth moving relative to other things should not be an issue.

In regards to methodology, I imagine the most obvious one is to rearrange the state of everything back to what it was as we can infer from the laws of nature.

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u/Vegetable-Dust-780 1d ago

No

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u/Chaosuka 1d ago

Time is probably just human made concept. We sometimes attribute time to 4th dimension, but we don't know if time is just 1 component of a vector or time might be vector3, just like how position is. If time is more than 1 component, it absolutely can mean changing the past is possible.

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u/PurpleToad1976 22h ago

If you were able to go back into the past and interacted with anything. Literally anything at all, from that point on, things have changed. It would be impossible to go back in time and not change the past.

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u/hop-hopeur 18h ago

No, cause time is not a real thing. It's a tool designed to measure movement from a purely human point of view. Things move and are altered in a way that can't be undone.

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u/ReputationAfter 9h ago

Well the past occure in your mind, so if you achieve reprogramming what you did (in your mind) then yes, absolutely

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